Re: [Qemu-devel] [PATCH v2 1/2] console: add hw_screen_dump_async

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/05/2012 12:22 PM, Avi Kivity wrote:
> On 03/05/2012 08:16 PM, Anthony Liguori wrote:
>>
>>>
>>>>     We're pretty close to being there.  Luiz, about how long do you
>>>> think before we get there?
>>>
>>> It's a pity to add new commands along the way.
>>
>> It's more complicated than this unfortunately.
>>
>> A client needs to be able to determine whether the 'screendump'
>> command works as expected.  Unfortunately, when QXL was introduced,
>> 'screendump' became broken across multiple releases.
>>
>> screendump is the right interface, but since it was broken in multiple
>> releases, we need another command for a client to determine that it's
>> not broken.  In the short term, screendump_async is that.  After QAPI,
>> it will probably be a screendump2.
>>
>> I don't mind introducing short term commands and then deprecating it
>> particularly when they are marked as such.
>
> Zooming out from screendump, there are several ways to deal with broken
> commands:
>
> 1. introduce new commands

This is our only short term options for this specific circumstance.

> 2. introduce capabilities that say "screendump is fixed wrt bug so-and-so"

We don't have such a mechanism and I generally would prefer to avoid it. 
There's a certain shame in introducing new commands that hopefully will cause us 
to be more careful in the future.

> 3. fix it and backport the fix to a stable release

This is only not practical because it requires such an infrastructure change.

> 4. ignore the broken command and hope

5. just tell clients to rely on version information and ignore the existence of 
downstreams

This is really mostly a downstream problem, not an upstream problem.  Version 
numbers can be reliable here for upstream.

This is the approach that would be typically taken for a C library.  The flip 
side is that this is also while we end up with autotools and a bunch of compile 
time tests to determine what broken functions exist.

Regards,

Anthony Liguori

>
> My preference is 3->2->1->4, or we'll end up with screendump65535 soon.



>